Joint separation of acetylene and ethylene from cracked gases

ABSTRACT

A gas mixture (obtained by thermal cracking of hydrocarbons followed by quenching of the cracked gas and freed from carbon dioxide and water) is separated into a gas mixture containing hydrogen, carbon monoxide and methane and another gas mixture containing ethylene, acetylene and higher hydrocarbons by (a) cooling the gas mixture which is at superatmospheric pressure in at least one condensation stage to a temperature which is above the solidification temperature of acetylene mixed with the other condensed constituents, (b) separating the condensate thus obtained and subdividing it in a rectifying column into a first mixture containing C2 hydrocarbons and a second mixture containing C3 and higher hydrocarbons, and (c) freeing the residual gas remaining from (a) from residual acetylene and ethylene in a scrubber by treatment with some of the mixture consisting of C3 and higher hydrocarbons from (b).

Nagel et al.

154] J QIN T SEPARATION OF ACETYLENE AND ETHYLENE FROM CRACKED GASESInventors: Otto Nagel, Hambach; Rolf Platz, Mannheim; Kurt Taglieber;Kurt Weinfurter; Dieter Wolf, all of Ludwigshafen, all of GermanyBadische Anilinit Soda-Fabrlk Aktiengesellschatt, Ludwigshafen/Rhein,Germany Filed: May 16, 1969 App]. No.: 825,323

Assignee:

Foreign Application Priority Data May 16, 1968 Germany ..P 17 68 460.5

us. Cl. ..62/17, 62/23, 62/28,

* 55/64, 260/679 A Int. Cl ..F25j 1/00, F253 3/00, F253 3/02 Field ofSearch ..62/23, 24, 27,28, 29, 39,

References Cited UNITED STATES PATENTS 6/1943 Felbeck .62/ l 7 11/1962Cost 12/1933 Wullf 1 H1938 Brewster ..62/39 8lillllillll PrimaryExaminer-Norman Yudkoff Assistant Examiner-Arthur F. PurcellAttorney-Johnston, Root, OKeefe, Keil, Thompson & Shurtleff 1 [57]ABSTRACT A gas mixture (obtained by thermal cracking of hydrocarbonsfollowed by quenching of the cracked gas and freed from carbon dioxideand water) is separated into a gas mixture containing hydrogen, carbonmonoxide and methane and another gas mixture containing ethylene,acetylene and higher hydrocarbons by (a) cooling the gas mixture whichis at superatmospheric pressure in at least one condensation stage to atemperature which is above the solidification temperature of acetylenemixed with the other condensed constituents, (b) separating thecondensate thus obtained and subdividing it in a rectifying column intoa first mixture containing C hydrocarbons and a second mixturecontaining C and higher hydrocarbons, and (c) freeing the residual gasremaining from (a) from residual acetylene and ethylene in a scrubber bytreatment with some of the mixture consisting of C and higherhydrocarbons from (b).

6 Claims, 3 Drawing Figures lllllilllili H PATENIEDJmwEm 3.635038 SHEET1 BF 2 FIG. I

a b c He, CO,CH

-80-IZOC V e f C FIG. 2

G) llllllllllll INVENTORS: OTTO NAGEL 28 ROLF PLATZ KURT TAGLIEBER KURTWEINFURTER DIETER WOLF ATT'YS PATENTEDJANIB-IBTZ- 3.635.038

SHEET 2 0F 2 mllllllllllll INVENTORSI OT O NAGEL ROLF PLATZ KURTTAGLIEBER KURT WEINFURTER DIE ER WOLF JOINT SEPARATION OF ACETYLENE ANDETIIYLENE FROM CRACKED GASES This invention relates to a process forseparating gas mixtures which contain hydrogen, carbon monoxide,methane, ethylene, acetylene and higher hydrocarbons, said mixtureshaving been obtained by thermal cracking of hydrocarbons followed byquenching of the cracked gas.

It is known from US. Pat. application Ser. No. 659,394 filed Aug. 9,1967, by Rolf Platz et al., now U.S. Pat. No. 3,471,584, that acetyleneand ethylene can be separated together from such gas mixtures by firstremoving carbon dioxide and hydrogen sulfide from the cracked gas, thendrying the gas, separating some of the methane and all the otherhydrocarbons by condensation and then separating the acetylene andethylene together with methane and ethane by pressure distillation froma liquid fraction which contains the whole of the C to C, hydrocarbonsand any carbon oxysulfide and carbon disfulfide.

This method of separation gives good results but has the disadvantagethat the gas mixture, after separation by condensation of thehydrocarbons with the exception of methane, still contains small amountsof ethylene and acetylene so that there is a risk that solid acetylenemay separate upon further cooling, for example by flashing.

It is therefore the object of this invention to provide a process whichdoes not have this disadvantage.

We have now found that gas mixtures containing hydrogen, carbonmonoxide, methane, ethylene, acetylene and higher hydrocarbons anddevoid of carbon dioxide and water, which have been obtained by thermalcracking of hydrocarbons followed by quenching of the cracked gas can beseparated into (i) a gas mixture containing hydrogen, carbon monoxideand methane which is practically devoid of other hydrocarbons and (ii) amixture containing ethylene, acetylene and higher hydrocarbons which maybe separated in further stages into the individual components, by aprocess wherein a. the initial gas mixture which is undersuperatmospheric pressure, preferably from 5 to 35 atmospheres gauge, iscooled in one or more condensation stages to a temperature which isabove the solidification temperature of acetylene in admixture withother condensed constituents, preferably to -82 to -l C.;

b. the condensate thus obtained is separated and subdivided in arectification column into a mixture containing the C hydrocarbons and amixture containing C and higher hydrocarbons; and I c. the residual gasmixture which still contains small amounts of acetylene and ethylene isfreed from acetylene and ethylene in a scrubber by treatment with someof the mixture containing C and higher hydrocarbons obtained in stage(b).

The term thermal cracking includes the conventional cracking methods inwhich a gas mixture containing ethylene and acetylene is obtained. Itincludes in particular the cracking of crude oil or hydrocarbonfractions in a flame burning beneath the surface of the liquid(submerged flame). This method is described in detail in Chem. lng.Technik 26, No. 5, page 253, and British Pat. No. 834,419, to whichreference is made.

Preliminary separation of carbon dioxide and hydrogen sulfide may becarried out in known manner by scrubbing, for example with an aqueoussolution of an alkali metal salt of an amino acid. This scrubbing, inwhich most of the carbon dioxide together with most of the hydrogensulfide is separated, may be followed by afterpurification from carbondioxide. This afterpurification is advantageously carried out byscrubbing with a dilute solution of an alkali metal hydroxide.

The cracked gas freed from carbon dioxide and hydrogen sulfide is driedbefore it is subjected to condensation. This drying may be carried outfor example by scrubbing or saturation with methanol. The cracked gasfreed from carbon dioxide and hydrogen sulfide and treated in this wayis cooled in one or more condensation stages to a temperature above thepoint at which acetylene separates as a solid, for example to from aboutto l20 C., the condensate containing C and higher hydrocarbons isseparated and the residual gas which contains acetylene and ethylene isfreed from acetylene and ethylene in a scrubber at superatmosphericpressure, advantageously at the pressure at which the cracked gas isformed, by scrubbing with the mixture of C and higher hydrocarbonsobtained as described below. This is shown in the simplified FIG. 1 (inwhich a denotes cracked gas, b cooler, c residual gas, d scrubber, econdensate, f laden scrubbing liquid, g distillation unit, and hscrubbing liquid) of the drawings which does not take into accountcountercurrent heat exchange. The gas mixture leaving the scrubbingcolumn is advantageously supplied for the production of cold to anexpansion machine for performing external work, in which it is expandedto practically atmospheric pressure, thereby cooled and then aftercooling the scrubbing liquid for the scrubbing column is used forcooling the incoming cracked gas countercurrently. The condensate andthe bottoms product of the scrubber are released from pressure and alsopassed countercurrent to the incoming cracked gas through thecondensation stages for cooling and condensation of the incoming crackedgas. The mixture of vapor and liquid formed from the condensate bypartial evaporation is'compressed to a pressure of from 10 to 20atmospheres and separated in a rectifying column at pressures of forexample 5 to 40 atmospheres gauge, preferably from 10 to 20 atmospheresgauge, into an overhead product containing C hydrocarbons andconstituents of lower boiling point, and a bottoms product containing Chydrocarbons and constituents of higher boiling point, and according toone embodiment of the invention (see FIG. 2) liquid ethylene obtained inthe subsequent separation of the overhead product into acetylene andethylene is supplied as a reflux. For further separation the overheadproduct is subjected for example to scrubbing with acetone. Some of themixture containing C hydrocarbons and constituents having a higherboiling point is withdrawn and serves as scrubbing liquid for theabove-mentioned scrubbing of the residual gas.

According to another embodiment of the invention, the mixture ofethylene andacetylene obtained is not separated into its constituentsbut used direct as a mixture for further reactions. For this purpose, inaccordance with FIG. 3, the condensate consisting of C to Chydrocarbons, after partial reevaporation, is separated in a rectifyingcolumn at a pressure of from 1.5 to 4 atmospheres absolute into amixture of ethylene and acetylene as overhead product and a bottomsproduct containing C to C hydrocarbons. In other respects, what has beensaid concerning the first embodiment applies in this case, too.

In order that the residual gas consisting of monoxide, hydrogen andmethane should be devoid of carbon disulfide, the scrubbing liquid mustnot contain any carbon disulfide which has not been removed from thecracked gas by the hydrogen sulfide scrubbing. For this purpose thescrubbing liquid consisting substantially of C and higher hydrocarbonsis advantageously not taken from the bottoms of the rectifying columnbut from a lateral stream withdrawn between the bottom and point ofsupply of the incoming mixture. In this way the sulfur compounds havinga higher boiling point than the said hydrocarbons (essentially carbondisulfide) are removed with the bottoms product. The scrubbing liquidwithdrawn as a lateral stream is passed through the heat exchanger ofthe condensation stages and cooled to a temperature of from about 80 tol20 C. and then supplied to the scrubbing column.

The invention is illustrated by the following examples.

EXAMPLE l A plant as shown diagrammatically in FIG. 2 is used. 10,000 m.(STP)/hour of cracked gas (obtained by an oxygen flame burning beneaththe surface of a pool of oil) which has been scrubbed free from carbondioxide and hydrogen sulfide and has been dried is introduced at apressure of 8.5 atmospheres absolute and a temperature of +20 C. throughline 1 into a heat exchanger 2. The cracked gas has the followingcomposition:

percent by volume CH,OH 0.69.

The cracked gas is cooled to C. and the condensate obtained is separatedin a separator 3. The residual gas which remains is passed through line4 through a heat exchanger 5 in which it is cooled to 1 10 C. and freedfrom condensate in a 20 separator 6. The residual gas has the followingcomposition (in percent by volume):

inert gas (11,, CO, CH 95.76 ethylene 2.78 acetylene 1 46 1t passesthrough line 7 into a scrubber 8 where it is substantially freed fromacetylene and ethylene by countercurrent treatment with a mixtureconsisting of 55 percent of C hydrocarbons, 39.5 percent of Chydrocarbons and 5.5 percent of C hydrocarbons introduced through line9. The residual gas leaving the scrubber 8 through line 10 at 1 12 C.

has an acetylene content of 50 p.p.m. and an ethylene content of 95p.p.m. it passes through line 10 to an expansion turbine 11 where it isexpanded from 8 to 1.5 atmospheres absolute and thus cooled to 163 C.This cold gas is used to cool, in

the intermediate coolers 13 and 14 of the scrubber 8, the solventflowing down therethrough, the gas thus being heated up to l35.5 C.Through line 12 it is passed through heat exchangers 5 and 2, giving upits cold therein and leaving the plant at a temperature of +10 C. Thesolvent containing acetylene and ethylene from the bottoms of thescrubber 8 is flashed together with the condensate from the separator 6through a valve into a line 15 and passed through heat exchanger 5 togive up its sensible heat and heat of evaporation. It is then mixed withthe condensate coming from separator 3 through line 16 and released frompressure. In order to give up its sensible heat of evaporation, thismixture is passed by line 15 through heat exchanger 2 which it leaves atabout 5 +10 C. Gas and liquid are separated in separator 17, thepressure of the liquid is raised by a pump 18 and supplied through anammonia-cooled heat exchanger 19 at 20 C. to a rectifying column 20. Thegas leaving separator 17 through line 21 is compressed by compressor 22to 18 atmospheres absolute, cooled to 25 C. with ammonia in heatexchanger 23 and also supplied to the column 20. Here the hydrocarbonmixture is separated into higher hydrocarbons and into a bottoms productcontaining sulfur compounds such as carbon disulfide. To maintain amixture of acetylene and ethylene which cannot be ignited by adetonator, liquid ethylene is supplied through line 24 and also gaseousethylene partly through line 25 from outside the plant into the column20. The amount of 3,500 kg./hour of solvent required for the scrubber 8is 5 withdrawn through line 26 from a tray lying between the bottoms andthe point of supply of the C -C hydrocarbon mixture, precooled withwater in heat exchanger 27, then after expansion to 8.5 atmospheresabsolute passed by line 9 through heat exchangers 2 and 5 and thuscooled to -1 10 C., and passed into scrubber 8. The mixture consistingof C hydrocarbons and higher hydrocarbons and a part of the sulfurcompounds and which has been condensed out and separated from thecracked gas entering the plant is withdrawn from the plant from thebottom of the rectifying column 20 through line 28.

The reflux necessary for rectification is produced with a condenser 29cooled with ammonia. The mixture of C hydrocarbons (consistingsubstantially of acetylene and ethylene and containing only a few p.p.m.of higher hydrocarbons) leaving the condenser through line 30 iswithdrawn from the plant, supplied to a scrubber and separated into theindividual components.

EXAMPLE 2 For the production of a mixture of acetylene and ethyleneaccording to P16. 3, 10,000 m. (STP) per hour of dried cracked gas (fromwhich carbon dioxide and hydrogen sulfide have already been removed) isintroduced through line 1 at a pressure of 8.5 atmospheres absolute anda temperature of +10 C. into a heat exchanger 2. The cracked gas has thecomposition given in example 1. After cooling to 20 C. in heat exchanger2, the condensate formed is separated in a separator 3. The gas iscooled to -1 10 C. in a heat exchanger 5 and freed from condensate in aseparator 6. The remaining gas has the composition given in example 1.Residual acetylene and ethylene is scrubbed out in scrubber 8 with a C Cfraction. Connection of the scrubber and expansion turbine 11 isidentical with the arrangement described in example 1. The condensatecollected in line 15 is heated to 0 C. in heat exchangers 5 and 2 toutilize sensible and evaporation heat. After cooling with the overheadproduct from column 20 in heat exchanger 31, the supply to the column 20is separated in a separator 17 into a liquid phase and a gas phase whichare supplied to different trays according to their composition.

Since according the procedure of this example the C mixture is notseparated, it is not possible to supply ethylene for the rectifyingsection of the column 20.

To avoid decomposition of acetylene the column is therefore operated atlow pressure (2 atmospheres absolute). Contrasted with the procedure ofexample 1, compressor 22 and pump 18 are not required here. The coldnecessary for the top condenser 29 is produced by an ethylenerefrigeration plant. The mixture of higher hydrocarbons obtained at thebottom of column 20 is withdrawn through line 28.

We claim:

1. A process for separating a gas mixture containing hydrogen, carbonmonoxide, methane, ethylene, acetylene and higher hydrocarbons which isdevoid of carbon dioxide and water and which has been obtained bythermal cracking of a hydrocarbon followed by quenching of the crackedgas, which process comprises a. cooling the initial gas mixture which isunder superatmospheric pressure in at least one condensation stage to atemperature which is sufficient to condense the C and higherhydrocarbons but which is above the solidification temperature ofacetylene in admixture with other condensed constituents;

b. separating and dividing the condensate thus obtained in a rectifyingcolumn into a first mixture consisting essentially of the C hydrocarbonsand a second mixture containing the C and higher hydrocarbons; and

c. freeing the residual uncond'ensed gas mixture of step (a) which stillcontains small amounts of acetylene and ethylene from said acetylene andethylene in a scrubber by treatment with some of said second mixturecontaining C and higher hydrocarbons obtained as a liquid condensatefrom step (b), thereby obtaining a separate third gas mixture whichcontains hydrogen, carbon monoxide and methane and which is practicallydevoid of other hydrocarbons.

2. A process as claimed in claim 1 wherein the initial gas mixture under(a) is at from 5 to 35 atmospheres gauge.

3. A process as claimed in claim 1 wherein cooling under (a) is to from82 to l20 C.

4. A process as claimed in claim 1 wherein the residual gas freed fromacetylene and ethylene and leaving the scrubber at a temperature of fromto 1 20 C. is expanded in an exevaporated in countercurrent heatexchange.

6. A process as claimed in claim 1 wherein a portion of the mixturecontaining C and higher hydrocarbons is withdrawn at a lateral pointbetween the bottom of said rectifying column and the point at which thecondensate to be separated is supplied.

2. A process as claimed in claim 1 wherein the initial gas mixture under (a) is at from 5 to 35 atmospheres gauge.
 3. A process as claimed in claim 1 wherein cooling under (a) is to from -82* to -120* C.
 4. A process as claimed in claim 1 wherein the residual gas freed from acetylene and ethylene and leaving the scrubber at a temperature of from -80* to -120* C. is expanded in an expansion machine while performing external work so that the gas mixture is cooled, the gas is used for intermediate cooling of the scrubbing liquid of the scrubber in one or more intermediate coolers and then used for countercurrent cooling in the condensation stages.
 5. A process as claimed in claim 1 wherein for cooling the gas mixture in the condensation stages, the condensate obtained is released from pressure and wholly or partly evaporated in countercurrent heat exchange.
 6. A process as claimed in claim 1 wherein a portion of the mixture containing C3 and higher hydrocarbons is withdrawn at a lateral point between the bottom of said rectifying column and the point at which the condensate to be separated is supplied. 